The present invention relates to a seal between sealing surfaces on inner and outer turbine casings for sealing between high and low pressure regions on opposite sides of the seal and particularly relates to arcuate leaf seal segments having a tab-and-groove arrangement along adjoining ends cooperable with the sealing surfaces for minimizing leakage flow through the seal.
In an advanced gas turbine design, an annular inner casing is mounted for radial and axial expansion and contraction relative to a surrounding annular outer casing. Each casing comprises a pair of generally semi-circular casing halves joined at their midline to one another. The inner casing mounts the first and second-stage nozzles and shrouds for the turbine, as well as ancillary parts for, among other things, carrying cooling circuits for the nozzles and shrouds. The outer casing is stationary and mounts the combustors and ancillary parts including for supplying the cooling medium to the inner casing, the shrouds and nozzle stages. Compressor discharge air at high pressure is supplied in a region forwardly of the inner casing and a portion of the outer casing for flow to the turbine combustors. A lower pressure region lies between the inner and outer casings and which region extends aft from an axial location along the turbine corresponding generally to the first bucket. It has been the practice to provide an annular seal between adjacent annular sealing surfaces on the inner and outer casings to seal between these high and low pressure regions.
Because of the relative radial and axial expansion of the inner and outer casings, it has been customary to provide a series of arcuate leaf seals carried by the inner casing in arcuate end-to-end relation relative to one another and overlying radially spaced, axially forwardly facing sealing surfaces of the inner and outer casings. This end-to-end sealing arrangement provides circumferentially extending gaps between end edges of each pair of adjacent leaf seals necessary to accommodate expansion and contraction of the casings and preclude arch binding of the seals. To minimize the flow through each gap, a cover plate overlies adjoining ends of the leaf seals along a surface of the seals opposite from the sealing surfaces of the inner and outer casings. While these leaf seals with cover plates have reduced the flow from the high to the low pressure regions, significant leakage remains. For example, with the seals engaging the radially spaced circumferentially extending sealing surfaces of the inner and outer casings and the cover plate overlying the gaps between adjoining ends of the seal plates, high pressure flow occurs between the overlying cover plate and the circumferential gap between registering end edges of the seal plates. Accordingly, there is a need to provide a seal which will further minimize flow between the high and low pressure regions on opposite sides of the seal plates.
In accordance with a preferred embodiment of the present invention, there is provided arcuately segmented seal plates for sealing against radially spaced, axially facing sealing surfaces of the inner and outer casings for eliminating or minimizing the leakage flow between high and low pressure regions on opposite sides of the seal. Particularly, the seal includes a plurality of arcuate segmented seal plates disposed in an annular array thereof about the rotor axis between the high and low pressure regions. The seal plates are preferably carried by the inner casing and have sufficient radial extent to seal against the annular sealing surfaces of both the inner and outer casings. At the registering adjacent end edges of the seal plates, there is provided a projecting tab on a first end of a first seal plate and a recess on a second end of the registering seal plate for receiving the tab. The tab has circumferentially extending, radially spaced sealing portions or edges which seal against the respective arcuate sealing surfaces of the inner and outer casings. The end edge of the tab, as well as the end edges straddling the tab at the first end of the first seal plate, are respectively spaced circumferentially from the edge at the base of the recess and the end edges straddling the recess at the second end of the second seal plate to provide circumferential gaps therebetween. These circumferentially offset and circumferentially extending gaps enable the seal plates to expand and contract in circumferential directions relative to one another without engaging or binding up one another.
To minimize or eliminate the leakage flow past this circumferential gap between adjacent arcuate seal plates, a cover plate overlies the first and second ends of the respective plates. The cover plate is secured, for example, by welding to one of the seal plates and overlies the adjacent seal plate spanning the gap between the plates. The reduced leakage flow is created by reducing the size of the flow path, together with a flow turn. That is, with the radially spaced edges of the tab engaging the sealing surfaces of the inner and outer casings, a reduced clearance between those edges and the radially spaced edges of the recess significantly reduces the leakage flow through the juncture of the seal plates between the high and low pressure regions. Concurrently, the circumferential gap between adjacent seal plates remains the same as in the prior seals, enabling the leaf seal plates to circumferentially expand and contract relative to one another.
In a preferred embodiment according to the present invention, there is provided in a turbine having radially spaced inner and outer casings and generally annular sealing surfaces, a seal for sealing across the sealing surfaces and between high and low pressure regions on opposite sides of the seal, the seal comprising first and second seal segments disposed in a generally common plane in end-to-end relation to one another, a first end of the first segment having a projection and a second end of the second segment in opposition to the first end having a recess for receiving the projection, the first and second ends having edges defining circumferentially extending gaps therebetween, the projection having radially spaced edges for sealingly engaging the respective sealing surfaces of the inner and outer casings and radially spaced edges of the recess in opposition to respective edges of the projection defining radial gaps therebetween each having a smaller gap width than the width of each of the circumferential gaps between the first and second ends.
In a further preferred embodiment according to the present invention, there is provided in a turbine having radially spaced inner and outer casings and generally annular sealing surfaces, an annular seal for sealing across the sealing surfaces and between high and low pressure regions on opposite sides of the seal, the seal comprising first and second elongated arcuate seal segments disposed in a generally common plane in spaced circumferential end-to-end relation to one another, a first end of the first segment having a circumferentially extending tab and a second end of the second segment in opposition to the first end having a circumferentially opening recess for receiving the tab, the first and second ends defining circumferentially extending gaps therebetween, the tab having radially spaced, circumferentially-extending edges for sealingly engaging the respective sealing surfaces of the inner and outer casings, radially spaced edges of the recess in opposition to the radially spaced edges of the tab defining radially extending gaps therebetween each having a smaller gap width than the width of each of the circumferential gaps between the first and second ends and a cover plate overlying the first and second ends and the gaps to minimize flow between the high and low pressure regions across the seal.